Apparatus for detecting foreign particles on a surface of a reticle or pellicle

ABSTRACT

An apparatus for detecting foreign particles on a surface of a reticle or pellicle comprises an optical system through which a light beam forms a light spot on the surface. The surface and the light spot are shifted relatively so that the light spot scans the surface. Light detectors receive light scattered by a foreign particle and directed in different directions, respectively, and generated photoelectric outputs by which the presence of the foreign particle is discriminated. Beam diameter and scanning speed are varied to provide the most effective operation for particles of different sizes.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to an apparatus for checking defectsinvolved or residing in a mask and/or reticle used for the manufactureof semi-conductor elements, and, in particular, for detecting foreignparticles stuck to the mask and/or reticle. More specifically, thepresent invention relates to an apparatus for checking a mask and/orreticle covered with a transparent film or pellicle to protect the maskand/or reticle against fine dust floating in the environment, or thetransparent film or pellicle itself.

Related Background Art

Apparatuses for automatically detecting foreign particles stuck to amask and/or reticle (referred to generically as "reticle" hereinafter)by scanning the reticle by means of light beams are already known, asdisclosed in the U.S. Pat. Nos. 4,468,120 and 4,610,541. Suchconventional apparatus checks the existence of the foreign particles,size of the particles and sticking condition of the particles, bylinearly scanning a surface of the reticle in x-direction by means of agalvano-mirror (vibration mirror) and the like, while directing a spotof condensed laser beam onto said surface at an incident angle of 10° to20°, by shifting the reticle in y-direction, and then by receiving thelight scattered laterally and rearwardly (among the reflected light) bymeans of a plurality of photoelectric detectors arranged in a spaceposition related specially with respect to the position of illuminationof the laser beam.

In such checking apparatuses, it has recently been desirable to enableeffective checking of a reticle with a thin film of polymeric material(referred to as "pellicle" hereinafter).

The pellicle is fixedly supported on the reticle by a supporting frameso that the pellicle covers the reticle leaving a space between thepellicle and a surface of the reticle, thereby preventing adhesion orsticking of the foreign particles on the reticle. When a projecting andexposing operation is performed by means of an exposing device makinguse of such pellicle, even if the foreign particles are stuck or adheredto the surface of the pellicle, an of the foreign particles is nottransferred onto a semi-conductor wafer which receives projected images,since the image of the foreign particles is not focused on the wafer.

However, when the size of the foreign particles stuck to the surface ofthe pellicle is relatively large, there would be patches on thesemi-conductor wafer (uneven exposure). Further, when some of theforeign particles stuck to a lower surface of the pellicle (i.e., asurface facing the reticle) drop from the surface of the pellicle ontothe reticle, even if they are not so large as to create the unevenexposure, the image of the dropped particles would be transferred to thesemi-conductor wafer.

Therefore, even if the pellicle is used, it is necessary to check orinspect the position and size of the foreign particles stuck to thepellicle, and further to judge or discriminate whether the foreignparticles are sticked stuck to an upper surface (remote from thereticle) of the pellicle or a lower surface (facing the reticle) of thepellicle.

However, such discrimination could not be obtained by the conventionalchecking apparatuses.

Further, when the reticle covered with the pellicle was checked by theconventional apparatuses without any additional devices, the inventorsof the present invention have found the following disadvantages.

Firstly, since the laser beam or light advances obliquely from thepellicle to the reticle, an incident angle of the laser beam would bechanged in accordance with the scanning positions, thereby changinglight intensity of the spot of the laser beam reaching the reticlesurface, due to the fact that light permeability of the pellicle isseemingly changed by the change of the incident angle of the laser, beamwith respect to the pellicle.

Secondly, since the photoelectric detectors are arranged inpredetermined space position, an angle formed between the pellicle and alight path of the scattered light sent from the foreign particles to thephotoelectric detectors is changed in accordance with the stickingposition of the foreign particles, and accordingly the scanning positionof the light spot, due to the fact that light permeability of thepellicle is seemingly changed by changing an incident angle of thescattered light with respect to the pellicle.

Accordingly, even if the foreign particle is stuck in the same position,there arises a problem that the sensitivity of the detection of theforeign particle between the reticle with the pellicle and the reticlewithout the pellicle is different.

Further, since the supporting frame for supporting the pellicle isarranged around a pattern area of the reticle, when the pellicle ismounted on the reticle, there arose a defect that a part of thescattered light from the foreign particles stuck to the reticle isinterrupted by the supporting frame so as not to reach the photoelectricdetectors. In order to correct the defect, a method in which after thereticle is checked, the reticle is rotated at an angle of 90° or 180°,and then the reticle is checked again can be adopted; however, thismethod is time, consuming and thus, is not desirable.

Furthermore, in the conventional apparatus for detecting the foreignparticles, a light beam having the same diameter has been used both forchecking the reticle surface and for checking the pellicle surface

In general, the sensitivity of the detection of the foreign particledepends upon the beam diameter of the light beam for scanning thesurface to which the foreign particle is stuck; the smaller the beamdiameter the higher the sensitivity of the detection of the foreignparticle, thus enabling the detection of the smaller foreign particle.On the other hand, the smaller the beam diameter, the longer the timefor evenly scanning the whole surface to be checked.

As described above, the foreign particles stuck to the pellicle are noteasily projected onto the semi-conductor wafer (in comparison with theforeign particles stuck to the reticle), even if the particles arerelatively large. In other words, an allowable size of the foreignparticle stuck to the reticle differs from that of the pellicle; thepellicle is permitted to have the larger particles than the reticle.

Accordingly, when the pellicle surface is checked by the above-mentionedconventional detecting apparatus, the foreign particles which aresmaller than those that need to be detected can also be detected, withthe result that there arose a problem that it took longer for thedetection of the foreign particles than it normally needed.

SUMMARY OF THE INVENTION

A main object of the present invention is to provide an apparatus forchecking a mask or reticle provided with a protection film or pellicle,which enables the high accuracy checking of a surface of the protectionfilm or pellicle and/or of a surface of the mask or reticle.

Another object of the present invention is to provide an apparatus fordetecting foreign particles stuck to a protection film or pellicle,which can discriminate or determine the position and size of the foreignparticles, as well as whether the particles are stuck to an uppersurface of the film or pellicle or a lower surface of the film orpellicle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are explanatory views showing scattered light beams fromforeign particles when a pellicle undergoes epi-illumination and obliqueillumination, respectively;

FIG. 3 is a perspective view showing a checking apparatus according to afirst embodiment of the present invention;

FIG. 4 shows outputs from a photoelectric detector;

FIG. 5 is a circuit block diagram of means for treating or conductingthe outputs from the photoelectric detector;

FIG. 6 shows a flow chart showing a main operation in the checkingapparatus according to the first embodiment of the present invention;

FIG. 7 is a perspective view showing a modification of the firstembodiment;

FIG. 8 is a perspective view showing another modification of the firstembodiment;

FIG. 9 is a perspective view showing a principle construction of ameasuring portion of a checking apparatus according to a secondembodiment of the present invention;

FIG. 10 is a perspective view showing a construction of the checkingapparatus according to the second embodiment of the present invention;

FIG. 11 is a circuit block diagram showing a construction of a signaltreatment circuit;

FIGS. 12A and 12B are graphs showing variation features of reflectivity;

FIGS. 12C to 12F are graphs showing correction features of amplificationdegree;

FIG. 13 is a perspective view showing a modification of the measuringportion;

FIG. 14 is a perspective view showing a checking apparatus according toa third embodiment of the present invention;

FIG. 15 is a plan view of the checking apparatus of FIG. 14;

FIG. 16 is a side view of the checking apparatus of FIG. 14;

FIG. 17 shows a signal treatment circuit;

FIG. 18 is a perspective view showing a checking apparatus according toa fourth embodiment of the present invention;

FIG. 19 is a circuit showing means for treating photoelectric outputs;

FIG. 20 is a perspective view showing a modification of the fourthembodiment of the present invention; and

FIG. 21 is a perspective view showing another modification of the fourthembodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be explained with reference to theattached drawings.

First of all, a checking apparatus according to a first embodiment ofthe present invention will be explained. The checking apparatusaccording to the first embodiment can discriminate whether foreignparticles are stuck to an upper surface of a pellicle or a lower surfaceof the pellicle, as well as the position and size of the foreignparticle.

An explanation will be given with regard to scattered light emitted fromthe foreign particles, when the foreign particles stuck or adhered toboth surfaces of the pellicle are illuminated from different directions.

FIGS. 1 and 2 show the condition or appearance of illumination andscattered light from foreign particles when a light-permeable pellicle 1to upper and lower surfaces of which foreign particles 2 and 3 are stuckis illuminated from different directions.

Particularly, FIG. 1 shows illumination light 4 and scattered light fromthe foreign particles 2 and 3 when the pellicle 1 is illuminated from adirection that forms a relatively large angle with respect to the planeof the pellicle (referred to as "epiillumination" hereinafter). On theother hand, FIG. 2 shows illumination light 5 and scattered light fromthe foreign particles when the pellicle 1 is illuminated from adirection that forms a relatively small angle with respect to the planeof the pellicle (referred to as "oblique illumination" hereinafter).

Firstly, the quantity of scattered light emitted from the foreignparticles received in a given direction, for example direction A shownin FIGS. 1 and 2 will be comparatively explained with respect to theepi-illumination and oblique illumination.

The quantity of scattered light emitted from the foreign particle 2stuck to the upper surface of the pellicle 1 under the epi-illuminationis substantially the same as that under the oblique illumination, if thequantity of the light of the epi-illumination is substantially the sameas that of the oblique illumination. On the other hand, with respect tothe foreign particle 3 stuck to the lower surface of the pellicle 1, thequantity of the scattered light from the foreign particle under theoblique illumination is less than that under the epi-illumination, dueto the fact that under the oblique illumination, since the illuminationlight 5 is reflected substantially completely on the upper surface ofthe pellicle 1, the quantity of light reaching the foreign particle 3 isless than that under the epi-illumination where a large amount of theillumination light passes through the pellicle 1.

Accordingly, it is possible to,discriminate whether the foreign particleis stuck to the upper surface of the pellicle or the lower surfacethereof, by illuminating the pellicle surface from two differentdirections and by comparing the quantity of the scattered light from theforeign particle obtained by such illumination from two directions.

Secondly, the quantity of scattered light emitted from the foreignparticles received in two different directions, for example directions Aand B shown in FIGS. 1 and 2 when the pellicle is illuminated from agiven direction will be explained.

With respect to the foreign particle 2 stuck to the upper surface of thepellicle 1, the quantity of the scattered light from the foreignparticle 2 received in the direction A is substantially the same as thatreceived in the direction B. On the other hand, with respect to theforeign particle 3 stuck to the lower surface of the pellicle 1, thequantity of the scattered light received in the direction B is less thanthat received in the direction A, due to the fact that the scatteredlight directed from the foreign particle 3 to the direction B is almostreflected on the lower surface of the pellicle 1.

Accordingly, it is possible to discriminate whether the foreign particleis stuck to the upper surface of the pellicle or the lower surfacethereof, by illuminating the pellicle from a given direction, byreceiving the scattered light from the foreign particle in two differentdirections and by comparing the quantity of the received light in saidtwo directions. Further, as shown in FIG. 2, when an incident angle ofthe illumination light 5 in the oblique illumination is so selected thatthe illumination light 5 is totally reflected on the pellicle surface,if there is no foreign particle stuck to the upper surface of thepellicle, since the oblique illumination light 5 is totally reflected onthe upper surface of the pellicle and the scattered light from theforeign particle 3 stuck to the lower surface of the pellicle is alsototally reflected on said lower surface, the difference in the quantityof light between the scattered light directed to the direction A in FIG.1 and that directed to the direction B can be more precisely clarified,which is desirable for the detection of the foreign particles.

In FIG. 3, the pellicle 1 is supported by a supporting frame 6 restingon a base plate 7 for the reticle or mask. The base plate 7 is fixed ona stage 8 which can be shifted independently to x-direction andy-direction in the illustrated x-y-z co-ordinate system by means of amotor (not shown).

A light source 10 for the oblique illumination is arranged laterally ofthe pellicle 1 and a light source 14 for the epi-illumination is alsoarranged above the pellicle. These light sources 10 and 14 may besources used with a conventional microscope and the like.

Light emitted from the light source 10 is converted to a substantiallycollimated light beam .by means of a lens 11 and then illuminates thepellicle 1 through a filter 12 through which a quantity of light iscontrolled. Similarly, light emitted from the light source 14 is alsoconverted to a substantially collimated light beam by a lens 15 and thenilluminates the pellicle through a filter 16 through which a quantity oflight is controlled.

A photoelectric detector 20 is arranged above the pellicle 1 andobliquely with respect to the pellicle. Scattered light emitted from theforeign particle is condensed by a lens 19 and is received by thephotoelectric detector 20, where the received light is converted to anelectric signal in proportion to the quantity of the received light.

An angle θ between the plane of the pellicle 1 and an optical axis 13 ofthe light from the light source 10 is selected in the order of 0° to10°. On the other hand, an angle between the plane of the pellicle 1 andan optical axis 17 of the light from the light source 14 may berelatively larger than the above-mentioned angle θ regarding the opticalaxis 13, and in practice may be selected in the order of 20° or more.

It should be noted that both of the light sources 10 and 14 are notnecessarily arranged on the same side of the pellicle 1: for example,the light source 14 can be arranged below the pellicle 1 or at the sideopposite to the light source 10 of the oblique illumination.

An optical axis 18 of the photoelectric detector 20 is so arranged thatthe detector 20 receives the scattered light from the foreign particlein the so-called dark field illumination by angularly deviating theoptical axis 18 from both the optical axes 13, 17 and optical axes oflight beams regularly reflected on the pellicle and emitted from thelight sources 10 and 14.

In operation, at first, the quantity of light reaching the pelliclesurface from the light sources 10 and 14 is previously adjusted by meansof the filters 12 and 14 for adjusting the quantity of light in such amanner that the quantity of the scattered light received by thephotoelectric detector 20 and emitted from the foreign particle stuck tothe upper surface (illuminated surface) of the pellicle under theepi-illumination is exactly the same as that under the obliqueillumination (that is to say, an output voltage of the photoelectricdetector 20 obtained by receiving the scattered light from the foreignparticle under the epi-illumination is the same as that under theoblique illumination). With respect to the illumination operation, forexample, the epi-illumination is firstly effected alone, therebydetecting the scattered light from the foreign particle under theepi-illumination by means of the photoelectric detector 20. Thereafter,the oblique illumination is effected alone, thereby detecting thescattered light from said foreign particle under the obliqueillumination. By repeating such illumination operations alternately fora predetermined period and by comparing the variation in the obtainedphotoelectric signals, it is possible to discriminate whether theforeign particles are stuck to the upper surface of the pellicle or thelower surface thereof.

As to the above-mentioned illumination operations in which theepi-illumination and the oblique illumination are effected independentlyor separately, one of the light sources may be deenergized orextinguished while energizing the other light source. Alternatively,both of the light sources can be continuously energized or lighted, butshutters can be provided on the optical axes of the light sources sothat only the light beam from one of the light sources reaches thepellicle by opening and closing the shutters selectively.

Next, features of the photoelectric signals obtained from thephotoelectric detector 20 will be explained with reference to FIG. 4.

FIG. 4 shows Variation A of "light intensity" I regarding "time" whenthe epi-illumination is effected by means of the light source 14;Variation B of "light intensity" I regarding "time" when the obliqueillumination is effected by the light source 10; Variation C of the"photoelectric voltage" V regarding "time" when the photoelectricdetector 20 receives the scattered light from the foreign particle 2stuck to the upper surface (illuminated surface) of the pellicle; andVariation D of the "photoelectric voltage" V regarding "time" when thephotoelectric detector 20 receives the scattered light from the foreignparticle 3 stuck to the lower surface of the pellicle 1, respectively.

When the epi-illumination and the oblique illumination are repeatedalternately at a predetermined cycle, as shown in the Variation C,variation or change (Vmax-Vmin) in the scattered light emitted from theforeign particle stuck to the upper surface of the pellicle will besubstantially zero. On the other hand, as shown in the Variation D,variation or change (Vmax-Vmin) in the scattered light emitted from theforeign particle stuck to the lower surface of the pellicle will berelatively larger than that in the Variation C; in this case, if theincident angles of the light beams from the light sources areappropriately selected, Vmin may be in the order of 50% or less of Vmax.

Therefore, by comparing the difference in the variation or change of thephotoelectric voltages, it is possible to determine the stickingdirection of the foreign particle, and thus, to discriminate whether theforeign particle is stuck to the upper surface of the pellicle or thelower surface thereof. More particularly, for example, it is possible todetermine the fact that the foreign particle is stuck to the uppersurface of the pellicle 1 when a standarized value(Vmax-Vmin)/(Vmax+Vmin) is less than a predetermined value (for example,0.33) and to determine the fact that the foreign particle is stuck tothe lower surface of the pellicle when said standarized value is equalto said predetermined value or more. Further, a rough size of theforeign particle can be judged from the value of Vmax.

Next, means for treating the outputs emitted from the photoelectricdetector 20 will be explained with reference to FIGS. 5 and 6.

FIG. 5 shows an example of construction of said treating means. In FIG.5, an actuator 31 for shifting the stage 8 is controlled by a controller32. The abovementioned light sources 10 and 14 can be energizedalternately by means of a driver 33 controlled by the controller 32. Thephotoelectric detector 20 is connected, through an amplifier 34, to alevel detector 35 which can detect both the above-mentioned maximumvalue Vmax and the minimum value Vmin. An output from the level detector35 is supplied to a comparator 36 and to an operator 37. In thecomparator 36, inputted data are compared with base data which havepreviously been statistically obtained and stored in the comparator andwhich represent the relationship between the sizes of the foreignparticles and the photoelectric voltages, thereby forming "size data"representing a rough size of each of the foreign particles.

On the other hand, in the operator 37, operations for finding out thesum (Vmax+Vmin) and the difference (Vmax-Vmin) of the maximum value Vmaxof the photoelectric voltage and the minimum value Vmin thereof areeffected.

An output of the operator 37 is connected to a discriminator 38. In thisdiscriminator 38, the abovementioned standarized value(Vmax-Vmin)/(Vmax+Vmin) is found out, and a magnitude relationshipbetween said standarized value and a pre-set criterion value o isjudged. It should be noted that said criterion value α is not limited toa specific value; for example, as explained with reference to FIG. 4,said value α may be in the order of 0.33. If the above-mentionedstandarized value (Vmax-Vmin)/(Vmax+Vmin) regarding a variation in thequantity of light is smaller than the criterion value α, it is judged ordiscriminated that the foreign particle is stuck to the upper surface ofthe pellicle 1, and if the standardized value is equal to or more thanthe criterion value α, it is discriminated that the foreign particle isstuck to the lower surface of the pellicle. The discriminator 38 outputs"direction data" which represent a sticking direction of the foreignparticle against the pellicle (i.e., represent the upper or lowersurface to which the foreign particle is stuck. The controller 32effects a necessary control relative to each of the above portions andgenerates positional data showing a position of a stage 8.

Next, an operation of the afore-mentioned means for treating thephotoelectric signals from the photoelectric detector 20 will beexplained with reference to a flow chart shown in FIG. 6. FIG. 6 showsan operation when the output signal is emitted from the photoelectricdetector.

Firstly, controlling instructions from the controller 32 are applied tothe driver 33, thereby energizing the light sources 10 and 14alternately. Next, other controlling instructions from the controller 32are applied to the actuator 31, thereby shifting the stage 8 in such amanner that the scanning of the pellicle 1 by means of the light beam iseffected at a speed slower than a period of theenergization-deenergization cycle of the light sources 10 and 14.

If the foreign particle is stuck or adhered to the pellicle, thescattered light from said foreign particle is received by thephotoelectric detector 20. The output voltage or photoelectric voltagefrom the photoelectric detector 20 is amplified by the amplifier 34 andthen is sent to the level detector 35, where the maximum value Vmax andthe minimum value Vmin are found out. Among these values, the maximumvalue Vmax is inputted or supplied to the comparator 36, where saidmaximum value is compared with the base data which have previously beenstatistically obtained and which represent the relationship between thesizes of the foreign particles and the photoelectric voltages(photoelectric signals), thereby finding out a rough size of saidforeign particle

Next, the maximum value Vmax and the minimum value Vmin found out in thelevel detector 35 are inputted to the operator 37, where the sum(Vmax+Vmin) and the difference (Vmax=Vmin) of these values are foundout.

These found values are inputted to the discriminator 38, where thestandarized value regarding the variation in the quantity of light VS(=(Vmax-Vmin)/(Vmax+Vmin)) is found out. And, it is discriminatedwhether the standarized value VS is smaller than the criterion value αor not. If the standarized value VS is smaller than the criterion valueα, the discriminator 38 generates direction data D1 which represent thatthe foreign particle is stuck to the upper surface of the pellicle, andif the standarized value VS is equal to or more than the criterion valueα, the discriminator 38 generates direction data D2 which represent thatthe foreign particle is stuck to the lower surface of the pellicle.

On the other hand, positional data of the detected foreign particleregarding x-y directions on the pellicle are outputted from thecontroller 32.

In place of the above-mentioned embodiment in which two light sources 10and 14 are energized alternately, a modification in which the wavelengthof the light from the light source 10 differs from the wavelength of thelight from the light source 14 thereby detecting the scattered lightemitted from the foreign particle on the basis of the wavelengths may beadopted. An example of such modification is shown in FIG. 7. In thisexample, wavelength selecting filters 41 and 43 are provided in place ofthe filters 12 and 16 in FIG. 3. Further, in place of the photoelectricdetector 20, two photoelectric detectors 47 and 48 are provided, and awavelength discriminating filter 46 such as a dichroic filter isinterposed between the detectors 47, 48 and the lens 19.

An example of a method for selecting or distinguishing the light on thebasis of the wavelength of the light by means of the above arrangementwill be explained. For example, the wavelengths of the light from thelight sources 10 and 14 are adjusted, by the wavelength selectingfilters 41 and 43, in such a way that the light from the light source 14can pass through the wavelength discriminating filter 46 but the lightfrom the light source 10 is reflected on the filter 46. With suchadjustment, the scattered light from the foreign particle created by thelight emitted from the light source 14 can be detected by thephotoelectric detector 47 but cannot be detected by the photoelectricdetector 48; on the other hand, the scattered light from the foreignparticle created by the light emitted from the light source 10 can bedetected by the photoelectric detector 48 but cannot be detected by thephotoelectric detector 47. With respect to the scattered light from theforeign particle stuck to the upper surface of the pellicle 1, theoutput voltage from the photoelectric detector 47 is substantially thesame as the output voltage from the photoelectric detector 48. On theother hand, with respect to the scattered light from the foreignparticle stuck to the lower surface of the pellicle 1, the level of theoutput voltage from the photoelectric detector 48 is less than that fromthe photoelectric detector 47.

Therefore, when the output voltage from the photoelectric detector 47 isS(47) and the output voltage from the photoelectric detector 48 isS(48), by comparing a value S(47)/S(48) with a predetermined value (forexample, 2), it is possible to discriminate whether the foreign particleis stuck to the upper surface of the pellicle or the lower surface ofthe pellicle. For example, when the value S(47)/S(48) is 2 or more thanthe predetermined value, it can be judged that the foreign particle isstuck to the lower surface of the pellicle, and when the valueS(47)/S(48) is 2 or less than the predetermined value, it can be judgedthat the foreign particle is stuck to the upper surface of the pellicle.

As described above, it should be understood that, in principle, if theposition of the illuminating system is replaced by the position of thephotoelectric system (light receiving system), the effects similar tothose in the above embodiments can be obtained. A checking apparatuswhich utilizes the above principle is disclosed in the U.S. Pat. No.4,468,120.

Next, a modification according to the above replacement will beexplained with reference to FIG. 8.

In FIG. 8, the light emitted from a light source 50 illuminates thepellicle 1 through a lenses 51. Scattered light from the foreignparticle is received by photoelectric detectors 56 and 59 throughfilters 54, 57 (for adjusting quantity of light) and lenses 55, 58,respectively. The light emitted from the light source 50 is converted toa substantially collimated light beam by means of the lens 51 and thenis sent to the pellicle 1 from the above and obliquely. Of course, theoblique illumination as shown in FIG. 2 is preferable.

A light receiving angle of the photoelectric detector 56, i.e., an angleθ_(A) formed between the plane of the pellicle and an optical axis ofsaid detector may be substantially the same as the aforementioned angleθ under the oblique illumination in the first embodiment, and,therefore, may be in the order of 0°-10°. Similarly, an angle betweenthe plane of the pellicle 1 and an optical axis of the photoelectricdetector 59 may also be substantially the same as the afore-mentionedangle under the epi-illumination in the above first embodiment, andthus, may be selected in the order of 20° or more.

In this modification (FIG. 8), since the ratio between the photoelectricoutput voltages obtained from two photoelectric detectors 56 and 59 whenthe foreign particle is stuck to the upper surface of the pellicle 1 issubstantially different from the corresponding ratio when the foreignparticle is stuck to the lower surface of the pellicle, it is possibleto determine the surface to which the foreign particle is stuck, bycomparing said ratios.

More particularly, at first, the filters 54 and 57 for adjusting thequantity of light are so adjusted that the output voltages S(56) andS(59) obtained from the photoelectric detectors 56 and 59 aresubstantially the same when they receive the scattered light emittedfrom the foreign particle stuck to the upper surface of the pellicle 1.Then, by comparing the output voltages S(56) and S(59) obtained from thephotoelectric detectors 56 and 59 when they receive the scattered lightfrom the foreign particle stuck to the lower surface of the pellicle 1,it is possible to discriminate whether the foreign particle is stuck tothe upper surface of the pellicle or the lower surface thereof. Forexample, if the ratio S(59)/S(56) is more than 2, it can be determinedthat the foreign particle is stuck to the lower surface of the pellicle,and if not, it can be determined that the foreign particle is stuck tothe upper surface of the pellicle.

It should be noted that when polarized light, particularlyS-polarization light (light polarized in a direction perpendicular tothe incident angle) is used as the illumination light, thediscrimination can be more easily and reliably effected. Moreparticularly, when the S-polarization light is supplied to the pellicleunder the oblique illumination, the quantity of light reflected on thepellicle will be greater than that in the case where a normal light (notpolarized) is supplied to the pellicle under the oblique illumination,with the result that the quantity of light passed through the pellicleand reaching the lower surface thereof will be very little Consequently,since the quantity of light of the scattered light from the foreignparticle stuck to the upper surface of the pellicle remarkably differsfrom the quantity of light of the scattered light from the foreignparticle stuck to the lower surface of the pellicle, by comparing suchquantities of light, the discrimination can be effected. For the reasonsabove, in the above-mentioned embodiments, it is not necessary to usepolarized light as to all of the illumination, but polarized light maybe used only in the case of the oblique illumination.

As the photoelectric detector, a CCD camera or a camera having an imagepickup tube (camera tube) can be used. By treating the photoelectricoutputs of the photoelectric detector obtained from the foreign particleby means of the treating means shown in FIG. 6, it is possible to knowthe sticking condition of the foreign particle.

Further, if the checking operation for discriminating whether theforeign particles are stuck to the upper surface of the pellicle or thelower surface thereof is performed over the whole area of the pellicle,it takes a long time to complete the operation; thus, in order toshorten or reduce the checking time, for example, the followingprocedures may be adopted; at first, the positions of the foreignparticles stuck to the pellicle are previously examined; secondly, saidpositions are recorded or stored (in an appropriate memory) asco-ordinate values (x,y) in the x-y-z co-ordinate system; then thepellicle is shifted to the position corresponding to each of saidco-ordinate values; and the checking operation is performed at theposition. In this way, effective checking can be effected.

Further, the foreign particle checking apparatus according to thepresent invention can be combined with a conventional optical microscopewhich can effect "epi-dark field illumination". In this case, a lightsource for illuminating the pellicle laterally at a small incident anglewith respect to the pellicle, and the associated lens and filter may beincorporated into the optical microscope. Further, in this case, byrepeating the epi-dark field illumination and the oblique illuminationalternately and by observing with eye (by means of the microscope) thevariation in the quantity of the scattered light from the foreignparticle, it is possible to discriminate whether the foreign particle isstuck to the upper surface of the pellicle or the lower surface of thepellicle.

Next, a second embodiment of the checking apparatus according to thepresent invention will be explained. The checking apparatus inaccordance with the second embodiment can automatically detect thepresence, size and/or sticking condition of the foreign particle, withthe same sensitivity of detection both in the case of the reticle (ormask) with the protection pellicle and in the case of the reticlewithout the protection pellicle.

As shown in FIG. 9, the checking apparatus includes a measuring portion60 for measuring permeability of a pellicle 61. A laser beam emittedfrom a laser source 62 which can output light having a wavelength thesame as a laser beam used in the actual surface checking operation issupplied to the surface of the pellicle 61 supported by a supportingframe 66 as the oblique illumination. A photocell or photoelectricdetector 63 receives the light passed through the pellicle. By rotatingthe pellicle by a motor about a straight line l₁ perpendicular to a linel₂ formed by projection of the laser beam on the pellicle, and then bymeasuring the photoelectric voltages of the photoelectric detector 63 asto the incident angles of the laser beam, the relationship between thepermeability and the incident angle (feature of variation of thepermeability) is measured.

When the pellicle 61 and the supporting frame 66 have already beenmounted on the reticle, it is difficult to directly measure thepermeability of the pellicle alone. Therefore, in this case, thereflected light is received by a photocell or photoelectric detector 64,the feature of variation of reflection factor is measured from thephotoelectric voltages of the detector 63, and the feature of variationof the permeability is presumed on the basis of said feature ofvariation of the reflection factor.

A range of the rotation of the pellicle about the line l₁ is preferablythe same as a range of the incident angle between the plane of thepellicle and the incident laser beam (from the laser source 62) or anangular range of the scattered light directed to the photoelectricdetector; however, it should be noted that the range of the rotation ofthe pellicle is not limited to the above, since, as to a light beamhaving a given wavelength, the feature of variation of reflection factorof said light can be measured within a certain range of the incidentangles thereof, and the feature of variation of the permeability thereofcan also be fully presumed if the information regarding the pellicle(that is, the information whether the pellicle is provided with areflection preventing film coated thereon or not) is obtained.

Further, the wavelength of the measuring laser beam is not necessarilythe same as that of the actually checking laser beam, since thepermeability of the checking laser beam can be presumed from thepremeability of the measuring laser beam. The light from the lightsource is not necessarily monochromatic light.

Furthermore, although in the present embodiment it is supposed that thepellicle has a uniform thickness all over, if the thickness of thepellicle is not uniform, the non-uniformity or variety of the pelliclecan fully be coped with by a method which will be explained later.Preferably, a numerical aperture (N. A.) and a spot size of themeasuring laser beam are the same as those of the laser beam of theactual checking apparatus, respectively, but are not so limited.

FIG. 10 shows a surface checking apparatus into which the measuringportion shown in FIG. 9 is incorporated. The laser beam emitted from thelaser source 62 is divided, by a beam splitter 70, into a laser beamdirected to the measuring portion 60 and a laser beam directed to asurface checking portion 80. The laser beam directed to the measuringportion 60 is further divided, by a second beam splitter 71, into twolaser beams, one of which is received by a photocell 72 for monitoringthe quantity of light. The other light beam from the beam splitter 71 isdeflected by a mirror 73 and then reaches the pellicle 61 arranged in apredetermined position in the measuring portion 60. The beam passedthrough the pellicle and the beam reflected on the pellicle are receivedby photoelectric detectors 63 and 64, respectively. As a function of theincident angle θ of the beam by dividing values of the quantity of lightreceived by the photoelectric detectors 63 and 64 by a value of thequantity of light received by the photocell 72, respectively, thefeature of variation of permeability of the laser beam and the featureof variation of reflection factor thereof are measured respectively. Inthis case, for example, when the reflected beam is measured, it ispreferable to rotate the photoelectric detector 64 about the axis orline l₁, as well as the rotation of the pellicle about said line, inview of reduction or minimization of the light receiver area. It shouldbe understood that, although not shown in the drawings, an appropriateoptical system (lenses and the like) is associated with or provided inthe measuring portion 60.

After the feature of variation of the permeability regarding thepellicle/incident-angle is measured by the measuring portion 60, thepellicle itself or the reticle provided with the pellicle is conveyedinto the surface checking portion 80 by means of an appropriateconveying device (not shown). In the illustrated embodiment, a reticle82 provided with the pellicle is checked. In the surface checkingportion 80, the laser beam from the beam splitter 70 is deflected by amirror 74 and then is reflected by a scanner mirror 83 which can berocked by means of a motor M to undergo a so-called "laser beamdeflection". The scanning laser from the scanner mirror 83 is focused ona locus to be scanned on the surface of the reticle 82 by means of anobjective lens 84 and a beam expander optical system (not shown). Thescattered light from the foreign particle stuck or adhered to thereticle surface is received by photoelectric detectors 87 and 89 throughlenses 86 and 88, respectively.

Here, since the laser beam is sent to the pellicle as the obliqueillumination, the permeability of the pellicle regarding the laser beamis smaller than that in the epi-illumination. Further, when the opticalsupplying system (the objective lens 84 and the like) is not a so-called"telecentric optical system", the permeability of the pellicle varies inresponse to a variation of the incident angle. Furthermore, since anangle between the plane of the pellicle and the beam of the scatteredlight directed to the light receiver optical system (lenses 86, 88)varies in dependence upon the scanning position of the laser beam, forexample, the quantity of light received by the photoelectric detectors87 and 89 and emitted from the foreign particle on the reticle 82 withthe pellicle differs from that in the case of the reticle without thepellicle, even if the laser beam is on the same scanning position.

Similarly, the quantity of light of the scattered light from the foreignparticle stuck to the lower surface of the pellicle 81, also varies, independence upon the scanning position of the laser beam. In this case,the variation of the quantity of light can be presumed on the basis ofthe thickness of the pellicle and the like. Since this information canbe obtained by the measuring portion 60, such variation can be easilycorrected upon detection of the foreign particle. Such correction of thevariation is not necessarily effected upon detection of the foreignparticle. For example, the correction can be performed by an operationin software, upon displaying the size and/or position of the foreignparticle as a checked result after the detection of the foreignparticle. In this case, the feature of variation of the permeabilityand/or the feature of variation of the reflection factor can be measuredafter the checking of the foreign particle. Further, when the surface ofthe pellicle 81 is checked by means of this apparatus, the locus to bescanned by the spot light of the laser beam is shifted onto the pellicleby lowering the reticle 82 by means of the stage (not shown) capable ofmoving in Z-direction.

FIG. 11 shows a block diagram for treating the photoelectric signalsgenerated in the apparatus of FIG. 10. A photoelectric signal 92 fromthe photocell 72 is inputted to a CPU 103 through a pre-treatmentcircuit 101 including a voltage transducer, an A/D converter and thelike. A photoelectric signal 93 from the photoelectric detector 64 forreceiving the reflected light in the measuring portion 60 is alsoinputted to the CPU 103 through a pre-treatment circuit 102 including avoltage transducer, an A/D converter and the like. At the same time, adata signal 94 which represents an angle converted from the rotation ofthe pellicle 61 about the line l is also inputted to the CPU 103 througha rotary encoder (not shown). After these data signals are inputted tothe CPU 103 sufficient to obtain the feature of variation of thepermeability of the pellicle regarding the incident angle for thedesired angular range, the CPU 103 creates or forms a table representingthe relationship between the permeability and the incident angle of thelaser beam (that is, the feature of variation of the permeability).Further, the CPU 103 finds out by an operation the permeability inresponse to the variation of the scanning position of the laser beam inthe x-direction, and sends the found data to a RAM 104 and a RAM 105.The RAM 104 stores the data regarding the signal from the photoelectricdetector 87, whereas the RAM 105 stores the data regarding the signalfrom the photoelectric detector 89. Therefore, when a value obtained bycounting time-series pulses from a pulse generator 110 is applied to theRAM 104 and RAM 105 as an address, the permeability at that scanningposition can be automatically read out.

The pre-treatment circuits 101, 102, CPU 103, RAM, 104 and RAM 105constitute an inputting means, whereas a correcting means is constitutedby multipliers 114 and 115, and voltage controlled amplifiers 118 and119. Since the Pre-treatment circuits 101, 102, CPU 103, RAM 104 and RAM105 form a portion for holding the information regarding the pellicle,said portion is referred to as "pellicle information holding portion100".

The scattered light from the foreign particle is received by thephotoelectric detectors 87 and 89 by which photoelectric signals 95 and96 corresponding to the quantities of light of the received light beamsare generated. These signals 95 and 96 are converted to voltages bymeans of voltage transducers 116 and 117, respectively. A waveformgenerator 111 creates, in synchronism with the output pulses of thepulse generator 110, a saw tooth wave 97 which is sent to a drivingcircuit for the scanner mirror 83. Therefore, by counting thetime-series pulses from the pulse generator 110, it is possible to knowthe scanning position of the laser beam in the x-direction. Thus, it isalso possible to determine the scanning position in the x-direction whenscattered light is being emitted, on the basis of the counted value.

When the reticle without the pellicle is checked, since the quantity oflight of the scattered light received by the photoelectric detectors 87and 89 varies in response to the variation of light receiving anglesthereof depending upon their positions in the x-direction, theabove-mentioned voltage controlled amplifiers 118 and 119 are providedfor adjusting gains with respect to each of signals from the voltagetransducers 116 and 117 to standardize the quantity of light. Anamplification degree conversion constant for this purpose is previouslyfound out by experiments and the like and is stored in ROMS 113 and 112.Thus, amplification degree conversion values can be sequentially readout from the ROMs 113, 112 in response to the time-series pulses fromthe pulse generator 110 and can be inputted to the voltage controlledamplifiers 118 and 119 through the multipliers 115 and 114, therebyeffecting the correction of the photoelectric signals regarding thequantity of the scattered light in accordance with the scanning positionin the x-direction. The other input constant of each of the multipliers115 and 114 is set or selected to "1" when the reticle without thepellicle is checked; in this case, values from the ROMs 113 and 112 areinputted to the voltage controlled amplifiers 118 and 119 just as theyare. When the reticle provided with the pellicle 81 is checked, thecorrected values regarding the scanning positions in the x-directionshould be further corrected or amended.

More particularly, if the position of the foreign particle in thex-direction is obtained, it is possible to determine the incident angleof the laser beam directed to the foreign particle. Further, theincident angles (with respect to the pellicle 81) of the light beams(among the scattered light from the foreign particle) directed to thephotoelectric detectors 87 and 89 can also be easily found out by thegeometrical positions of these photoelectric detectors. Thus, byinputting the time-series pulses from the pulse generator 110corresponding to the positions of the light spot in the x-direction onthe scanning locus to the RAM 105 and RAM 104 which store the dataregarding the feature of variation of the permeability (or thereflection factor) as the table representing the permeability (orreflection factor) with respect to the scanning positions, thecorrection values for the permeable amount of the scattered light inaccordance with the scanning positions of the laser beam in thex-direction can be sequentially obtained. In this way, since thecorrection values corresponding to the variation of the scanningpositions of the laser beam (light spot) in the x-direction due to theexistence of the pellicle are similar to the correction values stored inthe ROMs 113 and 112, by multiplying both of the correction values bymeans of the multipliers 114 and 115, and then by inputting the resultof multiplication to the voltage controlled amplifiers 118 and 119 asthe amplification degree conversion value in case of the checking of thereticle having the pellicle, it is possible to duly standardize thequantity of the scattered light, even if the pellicle exists. Each ofthe so standardized signals from the voltage controlled amplifiers 118and 119 is compared, by comparators 121 and 122, with each of referencevoltages (slice level) 123 and 124, respectively, thereby creating abinary mode with respect to the quantity of the scattered light having alevel larger than a predetermined signal level. Thus, when both of thequantities of the scattered light are larger than said predeterminedlevel, an AND circuit 125 outputs a digital signal 126.

Since the scattered light emitted from the foreign particle isdeflective, when data regarding a variation in the detection due to thedeflected components of the scattered light and the like have previouslybeen inputted to the CPU as various physical conditions, it is apparentthat the correction will be more accurate.

It should be understood that the correction means is not limited to theillustrated one; for example, the correction method can be performed byvarying the reference voltages 123 and 124 on the basis of theinformation from the pellicle information holding portion 100, or byvarying a displaying level when the information regarding the size andposition of the foreign particle are displayed on a CRT after thepermeability is measured in the measuring portion 60 and the foreignparticle is detected. As such displaying method, for example, a methoddescribed in the Japanese literature "PROCEEDINGS OF SPIE" (Vol. 470,published on March 14th and 15th, 1984) can be adopted. In this case, anoperation in the software corresponds to the correction method. Further,it is apparent that, if the feature of variation of the permeability ofthe pellicle has already been known, the measuring portion 60 may not beused, and thus the data regarding the permeability of the pellicle canbe inputted to the CPU before the checking operation is initiated.

Furthermore, when the data regarding the permeability inputted to theCPU are outputted to the RAMs 105 and 104, it is possible to use amethod in which some of estimated data have previously been inputted tothe CPU, then the data most nearly allied to data obtained by themeasuring portion 60 are picked up among the inputted data, then thepicked-up data are converted to the permeability corresponding to thescanning position (as correction factors), and lastly the correctionfactors are inputted to the RAMs 105 and 104. In this case, the datastored in the CPU and RAMs are provided or determined, for example, asones shown in FIGS. 12A-12F. Some of the data regarding the feature ofvariation of the reflection factor (in dependence upon the incidentangle) determined by the kind of the pellicles are stored in the CPU.Here, it is assumed that two data regarding the feature of variation ofthe reflection factor as representatively shown in FIGS. 12A and 12B arebeing stored in the CPU as the data base. In FIGS. 12A and 12B, theabscissa (horizontal axis) represents angle θ (deg.) between the planeof the pellicle and the incident light beam, and the ordinate (verticalaxis) represents the reflection factor. If it is judged, in the CPU,that the feature of variation of the reflection factor of the scatteredlight (received by the photoelectric detector 64) within the anglebetween 20° and 40° measured by the measuring portion 60 is most nearlyallied to the feature A shown in FIG. 12A, the CPU outputs the datacorresponding to correction curves A-1 and A-2 shown in FIGS. 12C and12D to the RAMs 104 and 105, respectively. In FIGS. 12C and, theabscissa represents the position of the light spot of the laser beam onthe scanning locus, and the ordinate represents the amount of gainadjustment applied to the multipliers 115 and 114. In FIG. 12C and 12D,the gain determined by the correction curve A-1 is given, for example,with respect to the photoelectric signal 95 from the photoelectricdetector 87; in the position x_(i) where the checking operation isinitiated, the correction is not effected (GAIN=1), whereas in theposition x_(f) where the checking operation is completed, the correctionis so effected that the amplification degree is substantially by twicelarger than that in the initial position (GAIN=2). On the other hand,FIG. 12D the gain determined by the correction curve A-2 is given withrespect to the photoelectric signal 96 from the photoelectric detector89; the correction is continuously effected so that the gain increasesto about 2 in,the initial position x_(i) and about 3 in the positionx_(f) where the checking operation is completed. Further, when the CPUjudges that the feature of variation of the reflection factor actuallymeasured is nearly allied to the feature B shown in FIG. 12B, the CPUoutputs the data regarding the correction curves B-1 and B-2 shown inFIGS. 12E and 12F to the RAMs 104 and 105, respectively.

It should be understood that more delicate corrections can be performedregardless of the quality and size of the pellicle, if the above database regarding the feature of variation of the reflection factor (orpermeability) or the gain correction curves (or features) are previouslyprepared in more details with respect to many kinds features.

FIG. 13 shows another embodiment of a measuring means for measuring thefeature of variation of the permeability (the relationship between thepermeability and the incident angle) of the pellicle, which correspondsto the measuring portion 60 of FIG. 10. In the measuring means shown inFIG. 13, it is assumed that the reflection factor of the pellicle ismeasured. A laser beam 131 reaches a scanner mirror 132 which can berotatably vibrated in the directions shown by an arrow, where the laserbeam is deflected. The deflected laser beam is refracted by a condenserlens 133 to focus at a point P on the pellicle 134 covering a reticle135.

Consequently, an angle between the plane of the pellicle and theincident laser beam at the point P varies between θ₁ and θ₂, and anangle between the plane of the pellicle and the regularly reflected beamalso varies between θ₁ and θ₂. The regularly reflected light beam isreceived by a photoelectric detector 337 through a condenser lens 336.Since an angle of the rotational vibration of the scanner mirror 132 isrelated to the incident angle of the incident laser beam on thepellicle, the feature of variation of the permeability (or reflectionfactor) of the pellicle 134 can be presumed on the basis of therotational angle of the scanner mirror and the photoelectric signal fromthe photoelectric detector 337. This embodiment has an advantage thatthe feature of variation of the permeability can be measured withoutrotation of a supporting frame for the pellicle and/or without movementof the photoelectric detector.

In this embodiment, too, the permeability may vary due to a thickness ofthe pellicle itself; thus, it is preferable to measure the permeabilityof the pellicle at different positions obtained by shifting the pelliclehorizontally along the line l₁ or l₂ shown in FIG. 10. In this case, ifthere is a difference in thickness in the scanning direction of thelaser beam in the surface checking portion 80 (FIG. 10), the correctionmay be effected by the afore-mentioned voltage controlled amplifiers 118and 119 by inputting data regarding such difference to the CPU 103 inthe same manner as mentioned above. Further, if there is a difference inthickness in the y-direction perpendicular to the scanning direction,the gains of the voltage controlled amplifiers 118 and 119 may beadjusted in synchronism with the movement (per unit) of the reticle inthe y-direction.

Furthermore, in the surface checking portion 80, the laser beam may besupplied from the reticle side (i.e., underside of the checking portion80) and the scattered light received at the pellicle side (i.e., upperside of the checking portion). Such construction can also be adopted inthe case where the reticle is covered on its both sides by thepellicles.

Further, the surface checking portion 80 shown in FIG. 10 can beconstructed integrally with the measuring portion 60. In this case, byconstructing the scanner mirror 83 so that it permits temporary stoppingof the light spot at the center of the scanning locus and by arrangingthe photoelectric detector 64 so that it can receive the regularlyreflected light, the feature of variation of the permeability of thepellicle can be measured as well. In the measuring portion 60, it is notnecessary to vary the incident angle of the laser beam on the pelliclecontinuously; by measuring the permeability of the pellicle with respectto at least two incident angles, it is possible to obtain thepermeability with respect to other incident angles by an operation onthe basis of the measured data.

In the above-mentioned second embodiment, the checking operationregarding the reticle having the protection pellicle positioned spacedapart from the reticle surface has been explained; however, the checkingoperation can be adapted to the checking of the reticle having aprotection glass plate affixed to a reticle surface on which a patternsuch as a chrome pattern is formed, as well.

Furthermore, in the above-mentioned embodiments, the sensitivity of thedetection was effected after the photoelectric detectors received thescattered light; however, the sensitivity of the detection can beeffected, for example, by providing an adjuster for adjusting thequantity of light (i.e., an attenuator) 140 such as an AOM (acousticoptical modulator) in a path of the laser beam between the beam splitteror half-prism 70 and the mirror 74 and by quickly adjusting the lightintensity of the laser beam scanning the reticle surface (or thepellicle surface) by an amount determined according to the scanningposition.

It is difficult to completely correct the variation of the sensitivityas to the measurement of the feature of variation of the permeability bymerely adjusting the light intensity of the laser light spot, when thelaser beam is supplied to the reticle surface in the obliqueillumination and a plurality of the photoelectric detectors are directedto the scanning locus on the reticle surface at different spacedpositions. However, by modulating the intensity of the light spot inaccordance with the scanning positions, it is possible to remarkablydecrease or reduce the amount of correction regarding the electricalsystem as to the photoelectric detectors, with the result that acomprehensive dynamic range regarding the detection of the foreignparticle can be enlarged. This means that the foreign particles havingvarious sizes can be checked with proper sensitivity. Even when thelaser beam is supplied as the oblique illumination, the correction canbe fully effected by merely modulating the light intensity of the lightspot according to as the position of the photoelectric detectors.

Next, a third embodiment of the present invention will be explained. Thechecking apparatus according to the third embodiment enables thechecking of the surface of the reticle without being obstructed by thesupporting frame for the pellicle.

In FIG. 14, a stage 208 for shifting a reticle 205 can be driven in they-direction by means of a motor 209, a lead screw and the like. Anamount of movement of the stage 208 is measured by a length meter(device for measuring the length) 210 such as a linear encoder. Apellicle 207 is mounted on the reticle 205 through a supporting frame206 for supporting the pellicle. An incident angle of a laser beam 201onto the reticle is selected in a range of 80°-10° so that the incidentlaser beam is not obstructed or interrupted by the supporting frame 206.The laser beam 201 can scan the reticle in the x-direction with the aidof a scanner mirror 203. In this case, the laser beam may be polarizedby a deflector 202 so that the incident beam is perpendicular to theplane of the reticle (i.e., x-y plane in FIG. 14) (S-polarization). Inorder to scan the whole area of the reticle by the laser beam, thescanner mirror 203 is vibrated and, at the same time, the stage 208 ismoved in the y-direction by means of the motor 209 at a speed lower thanthe scanning speed of the laser beam. The length meter 210 outputs ameasured result corresponding to the illuminating position (scanningposition) in the y-direction. With respect to the x-direction, theilluminating position is measured by a signal representing therotational angle of the scanner mirror 203. In order to detect thescattered light from the foreign particle stuck on the reticle,photoelectric detectors 213 and 222 are arranged in a plane which isperpendicular to the reticle plane and which includes a scanning line orlocus S and photoelectric detectors 216 and 219 are arranged in a planepositioned on the same side as,the mirror 203 with respect to thefirst-mentioned plane. In front of the photoelectric detectors 213, 216,219 and 222, condenser lenses 211, 214, 217 and 220 are arranged. Duringthe movement of the reticle in the y-direction, when the scanning beamapproaches the pellicle supporting frame 206, the reflected light fromthe upper and lower surfaces of the reticle illuminates the supportingframe, which would generate stray light. Slit plates 212, 215, 218 and221 are provided for preventing such stray light from the supportingframe from entering into the corresponding photoelectric detectors 213,216, 219 and 222. These slit plates are arranged in positions conjugatedwith the scanning line S on the reticle with respect to thecorresponding condenser lenses 211, 214, 217 and 220. Each of thephotoelectric detectors is arranged in contact with or close to thecorresponding slit plate so that the detector is situated immediatelybehind a slit of the corresponding slit plate. Alternatively, a relayoptical system (not shown) may be provided between each of the slitplates and the corresponding photoelectric detector to achieve the sameeffect as the above. It is more advantageous that the slit has awedge-shape having a wider upper end rather than an elongatedrectangular shape, since, in the light receiving system for receivingthe light from the oblique direction, as the scanning position varies,the width of the slit also varies in accordance with a transversalmultiplying factor of the corresponding condenser lens.

FIG. 15 is a plan view of the apparatus of FIG. 14, and FIG. 16 is aside view of the apparatus of FIG. 14 looked at from the right (FIG. 14)in the y-direction. In FIG. 15, the photoelectric detectors and 222 arearranged in the plane which includes the scanning line S and which isnormal to the plane of the reticle, and the photoelectric detectors 216and 219 are so arranged that azimuths φ₂ and φ₃ thereof are in the orderof 15°-80°, respectively. Further, as shown in FIG. 16, the fourphotoelectric detectors 213, 216, 219 and 222 are arranged in such amanner that angles θ₁, θ₂, θ₃ and θ₄ between the plane of the reticleand the optical axes of the corresponding condenser lenses 211, 214, 217and 220 are in the order of 10°-80°, respectively on the illuminationside (i.e., above the reticle) and that they are symmetrical with eachother in regard to a plane which includes the incident light beamreaching a center O of the scanning locus S and that they are includedin planes including the reflected light beams from the point O. Further,the condenser lenses 211, 214, 217 and 220 are arranged substantiallyequidistant from the center O of the scanning locus and are positionedso that the optical axes of these lenses cross the scanning line S (Inthe illustrated embodiment these optical axes cross each other on thecenter 0 of the scanning locus S).

Next, the detecting operation for detecting the foreign particle stuckto the reticle will be explained.

During the scanning of the reticle 205 by the laser beam, when the laserbeam illuminates an edge of a pattern regularly formed on the reticle,the scattered light is strongly oriented to some directions. Thus, inthis case, one of the four photoelectric detectors outputs a remarkablystrong output signal. On the other hand, when the laser beam illuminatesthe foreign particle, the scattered light from the foreign particle isdispersed over all the directions; thus, the four photoelectricdetectors output the photoelectric signals equal to or substantially thesame as each other. A circuit for detecting the foreign particle fromthese photoelectric signals will be explained below.

As shown in FIG. 17, the outputs of the four photoelectric detectors213, 216, 219 and 222 are amplified by means of amplifiers 223, 224, 225and 226, respectively. However, the output levels of the fourphotoelectric detectors are influenced by the position of the foreignparticle on the reticle. More specifically, the output level of thedetector positioned near the foreign particle is larger than that of thedetector positioned remotely from the foreign particle. Thus, it isnecessary to correct each of the output levels in accordance with theposition of the foreign particle.

To this end, amplification degree converters. such as voltage controlledamplifiers 227, 228, 229 and 230 where the amplification degree isvaried by the applied voltage are directly connected to the amplifiers223, 224, 225 and 226, respectively to adjust the amplification degreesby means of a controller 231. A position detector 204 outputs a signalcorresponding to the rotational position of the scanner mirror 203. Thecontroller 231 receives the signal from the detector 204 and outputs, onthe basis of previously stored values, voltages which are inputted tocorresponding VCAs in accordance with the scanning positions of thelaser beam. The amplification degrees of the VCAs 227, 228, 229 and 230are determined in accordance with the positions of the detectors 213,216, 219 and 222. By performing the correction in this way, it ispossible to obtain outputs depending only upon the size of the foreignparticle, regardless of the position of the foreign particle.

Thereafter, these outputs are compared with a reference level 236 bymeans of comparators 232, 233, 234 and 235, respectively, thereby beingconverted to a binary mode. This reference level 236 is not necessarily.constant all the time, but may be varied in synchronism with therotational vibration of the scanner mirror. Further, that differentreference levels may be inputted to the corresponding comparators. Afterthe outputs from the detectors 213 and 222 situated on an extension ofthe scanning line S are compared with the reference level, when at leastone of the compared results has an H level, an OR circuit 241 outputsthe H level. When the laser beam is scanning an area of the reticle nearthe supporting frame 206, one of the two detectors 213 and 222 cannotreceive the scattered light from the foreign particle, since thescattered light is interrupted by the supporting frame. However, if onlythe foreign particle exists on the scanning line, since at least one ofthe detectors 213 and 222 surely receives the scattered light from theforeign particle, the OR circuit outputs the H level representing the"presence" of the foreign particle. The output from the OR circuit 241and the outputs from the two remaining comparators 234 and 235 areinputted to an AND circuit 243. The AND circuit 243 outputs a foreignparticle detecting signal only when the laser beam illuminates theforeign particle on the reticle. In order to obtain a rough size of theforeign particle without the influence of the scattered signals from thepattern formed on the reticle, it is effective to compare the intensityof the smaller signal among the photoelectric signals from the detectors216 and 219 with prearranged data regarding the relationship between thesize of the foreign particle and the intensity of signal. Such method isdisclosed in U.S. Pat. No. 4,610,541. In the present embodiment, suchmethod can be adopted.

More particularly, the outputs from the VCAs 229 and 230 are inputted toa comparator 252 and a switching device 253. The comparator 252 actuatesthe switching device 253 in such a manner that among the outputs fromthe VCAs 229 and 230 the smaller one is selected. The selected smalleranalogue signal is inputted to an A/D converter 255 to be converted intoa digital signal. By comparing the output signal from the A/D converter255 with statistically prearranged data representing the relationshipbetween the size of the foreign particle and the input signal, by meansof a computer (not shown), it is possible to know a rough size of theforeign particle.

Next, a fourth embodiment of the present invention will be explained.The fourth embodiment enables checking the foreign particle withsensitivity of detection (i.e., diameter of the scanning beam) accordingto the allowance as to the size of the foreign particle and at a speedcorresponding to said beam diameter, by changing the beam diameter onthe reticle in response to the foreign particle.

In FIG. 18, a table or stage 308 for supporting a reticle 305 providedwith a pellicle 307 can be shifted in the y-direction by means of amotor 309 (as in the case of FIG. 14) and can also be shifted in thez-direction by a driving means (not shown).

A laser beam 301 is adjusted to have any beam diameter by an expander302 and the like and then is supplied the reticle or the pelliclethrough a scanning mirror 303. Three sets of detecting assemblies311-313, 314-316 and 317-319 are arranged in spaced positions in thesame manner as the detecting assemblies 211-213, 214-216 and 217-219 ofFIG. 4. An aperture member 360 can be inserted in a path of the laserbeam between the expander 302 and the scanner mirror 303. When theaperture member 360 is inserted in the above-mentioned path, a numericalaperture of the optical system for the incident laser beam decreases.Consequently, the beam diameter of the laser beam on a focused point(i.e., on the reticle or on the pellicle) increases. Since a scanningwidth in the y-direction which is scanned by a single scanning operationperformed by the laser beam moving in the x-direction can be enlarged inaccordance with the increase of the beam diameter, when the beamdiameter is increased, the speed of movement of the stage 308 can befaster.

Therefore, when a pellicle having a great allowance as to the size ofthe foreign particle stuck thereto is checked, it is preferable toenlarge the beam diameter of the scanning laser beam and to increase thespeed of movement of the stage, thereby considerably shortening thescanning time for scanning the whole area of the pellicle, in comparisonwith the scanning time by means of the laser beam having a normal beamdiameter.

In FIG. 19, since each of the outputs from the photoelectric detectors313, 316 and 319 varies in accordance with a distance between the laserspot and each detector, these outputs are corrected in accordance withthe position of the foreign particle on the basis of prearranged data bymeans of voltage controlled amplifiers 327, 329 and 330 under thecontrol of a controller 331, as in the case of FIG. 17. The correctedoutputs are compared with a reference level 336 by means of comparators332, 334 and 335, respectively; when the output of the comparator has Hlevel, a detecting signal is generated in an AND circuit 343. The sizeof the foreign particle is determined by utilizing each output SA, SB,SC from each of the VCAs (voltage controlled amplifiers) 327, 329 and330. More particularly, at first, the relationship between the size ofthe foreign particle and the signal level of the smallest output amongthe outputs SA, SB and SC is statistically prepared previously; then, bycomparing the data regarding said relationship with an actually obtainedoutput signal, a rough size of the foreign particle can be obtained.

When the aperture member 360 is used for increasing the beam diameter,since the numerical aperture decreases and the quantity of light of thelaser beam also decreases, it is desirable to adjust the comparisonsystem by uniformly changing the amplification degrees of the VCAs 327,329 and 330 by means of the controller 331, or by changing the referencevoltage 336. With this adjustment, even when the beam diameter ischanged, the sensitivity of detection (determination of the size of theforeign particle and the like) regarding the foreign particle ismaintained in an optimum condition in accordance with the beam diameter.In place of the aperture member 360, two expanders 402 and 404 havingdifferent multiplying factors and change-over mirrors 406 and 408 forchanging over paths of a laser beam 401 (FIG. 20) may be provided forchanging the beam diameter of the laser beam. When the change-overmirrors 406 and 408 are out of the path of the laser beam, the laserbeam 401 is enlarged by an expander 402 and then is sent to the scannermirror 303. When the change-over mirrors 406 and 408 are inserted in thelight path of the laser beam simultaneously, the laser beam 401 isenlarged or expanded by a second expander 404 and then is sent to thescanner mirror 303. As a modification of the above construction, forexample, an expander having a variable multiplying factor and comprisinga zoom system may be used in place of the expanders 402 and 404.Further, as shown in FIG. 21, a focus of the incident laser beamilluminating the reticle may be slid or shifted, thereby changing thebeam diameter of the incident laser beam. When the foreign particlestuck to the pellicle is checked, the beam diameter of the laser spot onthe pellicle is changed by shifting one of lenses (for example, a lens302A) of the expander 302 on the light path without altering theposition of the stage 308 in the z-direction, thereby altering the focusposition of the laser beam.

The checking position where the reticle is checked differs from thechecking position where the pellicle is checked due to the presence ofthe supporting frame. Therefore, it is necessary to determine or selectthe widths of the slits of the slit plates 312, 315 and 318 so that thescattered light is not interrupted by the slit plates.

Of course, it is possible to change the beam diameter of the laser spoton the pellicle by shifting the stage 308 in the z-direction withoutaltering the focus position of the laser beam (that is, without shiftingthe lenses of the expander 302).

What is claimed is:
 1. An apparatus for detecting foreign particles on asurface of a reticle or pellicle, comprising:a supplying means forsupplying a light beam which forms a light spot on said surface; ascanning means for shifting said surface and said light spot relativelyso that said light spot scans the surface; a plurality of lightdetectors for receiving light scattered by a foreign particle on saidsurface and directed in different directions, respectively, and forgenerating photoelectric outputs, respectively; changing means forchanging the diameter of said light beam; and a discriminating means fordiscriminating the presence a foreign particle on said surface on thebasis of each of said photoelectric outputs.
 2. An apparatus as setforth in claim 1, wherein said discriminating means includes a comparingmeans for comparing each of said photoelectric outputs with a referenceoutput level, and a correcting means for correcting said comparing meansby changing said photoelectric outputs and said reference output levelrelatively in accordance with said changing means.
 3. An apparatus asset forth in claim 1, wherein said scanning means changes speed of saidshifting in accordance with changing of said beam diameter.
 4. Anapparatus as set forth in claim 1, wherein said changing means includesan aperture diaphragm retractable with respect to said beam.
 5. Anapparatus as set forth in claim 1, wherein said changing means includesa plurality of beam expanders having different expansions and means forselectively passing said light beam through any one of said beamexpanders.
 6. An apparatus as set forth in claim 1, wherein saidchanging means includes variable expansion beam expander means.
 7. Anapparatus as set forth in claim 1, wherein said supplying means includesa light source and an objective optical system fixed in said apparatusand arranged opposite to said surface, wherein said light beam istransmitted from said light source through said objective optical systemto said surface, and wherein said changing means comprises optical meansbetween said light source and said objective optical system for changinga numerical aperture of said supplying means.